Always check the easy stuff first, rather than jump straight to the most involved possibilities. The radio was replaced by the dealer 6 weeks ago. While it's not beyond the realm of reason that the refurbed unit they installed wasn't fixed correctly before it found it's way to your bike, it's just as likely that a poorly made connection came loose. Remove the outer fairing and check all the connections. If I remember right, which I probably don't, there is a separate harness and connector for the outboard amp. If that wasn't connected properly, I could see not having speakers but the headset output working.
I'm definitely no expert on these radio's, but with all that I've been reading about the HK's I don't think they or H-D are either. It appears to me that they took something that might be OK sitting in an entertainment center and tried to convert it to a motorcycle radio. Last time I checked my entertainment center didn't vibrate enough to shake the fillings in my teeth, unlike what happens in that fairing on a Harley. The fact that these problems have continued for several years now tells me that once again H-D isn't planning to fix anything right, they'll just keep replacing them with refurbs until the warranty is gone and then you're on your own. They really don't care about the ESP expense, that's a different company's headache. My point is, just like all the other stuff that we wind up fixing on our own, it sounds like a permanent fix for the HK crap will require money from our pockets. I have no idea what Iron Cross charges for their upgrade, but if you compare it to paying $50 every time the dealer takes out one POS and replaces it with another POS under the ESP, I imagine the cost becomes more reasonable and justifiable. As much as I hate letting the MoCo off the hook and paying to fix their crap, there comes a time when it just makes sense to cut your losses.
Jerry